1. Field of the Invention
The present invention relates to a surface lighting device and a display having such a lighting device. More particularly, the invention relates to a surface lighting device preferably used for a backlight device for backlighting a liquid crystal display and other displaying apparatuses, and a display having such a surface lighting device.
2. Related Background Art
A surface lighting device is capable of emitting light evenly over a wide area. It is thus applicable as various lighting means.
Particularly, since a liquid crystal itself does not emit any light in the liquid crystal display or the like, it is usually practiced to light up the liquid crystal panel from its rear side using a light source in order to improve the display conditions for an easier observation of the displayed images.
For a desirable surface lighting device serving as a backlight source of the kind, it is required and the luminance is even for the light emitting surface, that is, at least the area corresponding to the entire display surface of the display, and that the apparatus is small and light in weight as well as the lighting efficiency is high for its power consumption.
Also, when this surface lighting device is adopted as a display, its mode should preferably be such that the display can be fabricated thin as a whole.
As a surface light device which can satisfy such requirements, there is known the surface light device of an edge type using semiconductors, and as a display having such a surface lighting device, there is known the display which is represented in a schematically cross-sectional view in FIG. 1.
In FIG. 1, reference numerals 1a and 1b designate light sources; 2, a reflection board; 3, a diffusion board; 4, a light guide; 5, a liquid crystal display panel; 6, an outer housing; and 8, a diffusion reflection pattern.
FIG. 2 is a plan view schematically showing a surface lighting device used for the display shown in FIG. 1, which is observed from the light emitting side (from the side where the liquid crystal display panel is mounted).
In FIG. 2, a reference numeral 9 designates lamp leads; 10, lamp electrode portions; 11, a lamp emission portion; and 12, the effective emission surface of the surface light device.
FIG. 1 is a cross-sectional view schematically showing the display taken along 1--1' in FIG. 2.
The surface lighting device shown in FIG. 1 and FIG. 2 is provided with the light sources 1a and 1b with the light guide 4 therebetween. Also, the diffusion reflection pattern 8 is provided on the side opposite to the light emitting surface of the light guide 4. Then, there is provided the reflection board 2 which has an opening encircling the aforesaid light sources 1a and 1b, light guide 4, and diffusion reflection pattern 8. Also, the diffusion board 3 is arranged above the opening of the reflection board 2. The outer housing 6 is arranged to encircle the reflection board 2 with its opening portion provided corresponding to the opening of the aforesaid reflection board 2.
For the light guide 4, the above-mentioned surface lighting device has a transparent member made of a glass or plastic material such as acrylic resin, and the incident rays of light from the light sources 1a and 1b through the sides of the light guide 4 are emitted from the front face of the light guide 4 after being uniformalized through the reflection board 2 and diffusion board 3.
The light sources 1a and 1b are arranged at both ends of the effective light emission surface 12 to place it between them. The luminous fluxes emitted from the light sources 1a and 1b enter the light guide 4 made of transparent acrylic resin or the like. Then, the structure is arranged so as to guide them to the center as much as possible through the reflection pattern portion 8 on the reverse side of the light guide 4 and the reflection board 2.
Also, the diffusion reflection pattern portion 8 is arranged more densely further away from the light source 1a or 1b. It is thus attempted to guide the incident rays of light to the central portion of the light guide 4.
In this respect, FIG. 3 is also a cross-sectional view schematically showing the display having the surface lighting device taken along the same position as FIG. 1. In FIG. 3, what differs from FIG. 1 is that the light guide 4 has a curvature on each side of the light sources 1a and 1b to match the configurations of the light sources 1a and 1b. In this way, it is implemented to enable the rays of light from the light sources 1a and 1b to enter the photoconductor more effectively as well as to make the device compact.
However, as described above, the surface lighting device of an edge type has a low luminance in the central portion between the light sources and a high luminance in the vicinity of the light sources as indicated by a broken line C shown in FIG. 9. This is because the light sources 1a and 1b emit diffusion light and make the vicinity of the light sources 1a and 1b bright while the light emitted from the light sources 1a and 1b mostly reach the opposite light source 1b and 1a to be diffused, respectively, thus making the vicinity of the light sources 1a and 1b brighter. As a result, it is inevitable that the effective light range (effective emission surface) of the foregoing lighting device will become narrower because its overall luminance must be adjusted to latch evenly as a backlight with the lowered luminance between the central portion between the light sources 1a and 1b. Thus, a problem is encountered that the light utilization efficiency for the apparatus as a whole is reduced.
Also, in order to intensify the luminance in the central portion between the light sources, there are disclosed in Japanese Utility Model Laid-Open Application No. 63-8703 and Japanese Patent Laid-Open Application No. 01-91955 methods to guide luminous fluxes to the central portion by providing a lower recess or an upper recess for the central portion of the photoconductor as shown in FIG. 4A and FIG. 4B. According to these methods, it is possible to intensify the luminance of the back lighting device 1 for the central portion between the light sources as indicated by the dotted line B in FIG. 9. However, since the configuration of a photoconductor of the kind is uneven, it takes a long period of time to complete a molding cycle when it is molded without any adverse effects such as warping. The larger the light guide, the longer becomes such a period of time. In consideration of productivity, these methods are not realistic. Also, when the light guide is produced by cutting, there are still the problems of warping and unevenness of grinding finish among others. The machining which can avoid them is costly and this is not realistic, either.
In the conventional example, therefore, it is required to use a light source having all powerful luminous fluxes to intensify the luminance all over the backlight source. Accordingly, the light source itself becomes large, resulting in a thick liquid crystal display or the increased power consumption requires more capacitance to necessitate the use of larger light source. The heat generation of the light source itself becomes great to make it necessary to provide an additional heat releasing arrangements for the liquid crystal display. There are some other problems. Moreover, in recent years, there are demands on the larger panel of liquid crystal display or color liquid crystal display which has problem with transmittivity. To meet these demands, the above-mentioned problems become more significant and serious.
Also, in order to increase the light emission area more or intensify the emission luminance more for the expansion of the display area, the implementation of color display, or a better visivility of display, it is conceivable to provide the light sources 1a and 1b not only on one opposing sides of the light guide 4, but also on the other opposing sides thereof. Now, in conjunction with FIG. 5 and FIG. 6, such an example will be described.
FIG. 5 is a cross-sectional view schematically showing a display as in FIG. 1. Also, FIG. 6 is a perspective view schematically showing the relations between the arrangements of the light sources 1a, 1b, 1c, and 1d and the light guide 4. In this respect, FIG. 5, illustrates the display sectionally at a position corresponding to the position taken along 5--5' in FIG. 6. In FIG. 5 and FIG. 6, the same reference numerals as in FIG. 1 designate the same elements. Here, therefore, descriptions will be omitted for such elements.
In FIG. 5 and FIG. 6, the four light sources 1a to 1d are arranged along the four sides of the light guide 4 in a configuration of a board. However, if there is no diffusion reflection pattern portion 8 present, most of the luminous fluxes emitted from the light source 1a on the left-hand side in FIG. 5, for example, repeat critical reflections. These fluxes are guided to the liquid crystal display panel 5 side while repeating diffusion reflections or secondary reflection due to the surface of the opposing light source 1b side and are attenuated. Also, even if a diffusion reflection pattern 8 is provided for the reverse side of the light guide 4, most of luminous fluxes are guided to the liquid crystal display panel 5 side due to diffusion by the diffusion board 3 in the vicinity of the light source 1a or are diffused by the surface of the light source of the light source 1b side to be guided to the liquid crystal display panel 5 side. As a result, the luminance is low on the upper panel surface side in the vicinity of the central portion of the surface lighting device between the opposing light sources 1a and 1b. In addition, there is a problem that the backlight becomes uneven having a high luminance in the vicinity of the light sources.
In other words, as shown in FIG. 5, the luminous flux Z is not related to the diffusion reflection pattern portion 8. This reaches the light source 1b on the opposite side to increase the luminance in the vicinity of the light source 1b. Also, the light flux Y is substantially reflected critically (light fluxes less than the critical angle being totally reflected) on the upper surface of the light guide 4. Thus, this is not related to the diffusion reflection pattern portion 8, either. Therefore, it causes the luminance to be increased likewise in the vicinity of the light source 1b. In other words, in a usual backlight device of an edge type, it is difficult to intensify the luminance in the central portion between the light sources 1a and 1b according to FIG. 5. Accordingly, the device is designed to intensify the luminance in the central portion between the light sources 1a and 1b by the application of the diffusion reflection pattern portion 8 provided for the reverse side of the light guide 4. For the same reasons as described earlier, it is still impossible to intensify the luminance in the central portion between the light source sufficiently. The light emitted from the light source 1a reaches the light source 1b on the opposite side. It is thus diffused by the end face of the light guide 4 on the opposite light source 1b side to make the vicinity of the light source 1b brighter; hence creating a significant difference in luminance in the central portion between the light sources and the vicinity of the light sources.
In this case, in order to control the luminance in the portion near the light sources where the luminance becomes high, it may be necessary to elongate further the distance from the effective display surface to the light sources 1a and 1b. The ratio (effective efficiency of the light sources) to the total luminous fluxes which can be utilized on the panel face side against the entire luminous fluxes of the light sources becomes smaller still. Therefore, powerful light sources having all luminous fluxes should be employed to intensify the luminance of the backlight sources as a whole. Then, many problems are encountered such as the increased thickness of the entire body of the liquid crystal display due to the light sources themselves which are inevitably made larger, the necessity of a larger capacitance attributable to the increase of the power consumption, the increase amount of heat generated by the light sources themselves, and the necessity to countermeasure the heat generation of the liquid crystal display. In addition, the transmissivity is lowered as the liquid crystal display panel is made larger or the adoption of color display. These problems described above are now more significant and serious.